The invention pertains to the field of water recovery from aqueous solutions. More particularly, the invention pertains to the desalinization of a saline solution.
The economically and energetically practical desalinization of saline solutions has long been a goal of organized society. In fact, there are suggestions found in the Bible, and in writings by the ancient Greeks, that suggest knowledge regarding the desalting of brackish waters. The benefits of such a process are obvious and important in a world with both a rapidly increasing population and a relatively static fresh water supply. However, it was not until the nineteenth century in England that the first official studies were made into the possibility and practicality of desalinization and ion exchange.
Milestones in the scientific approach to desalinization include the realization by the German chemist Eichorn, in 1858, that ion exchange reactions are reversible ones. By 1905 German scientist Gans developed a process of softening water on a commercial basis using cation exchange materials. In 1935 two English chemists, Adams and Holmes, produced an ion exchanger. The cation exchanger used was a phenolformaldehyde condensation product and the anion exchanger was a condensation product of polyamines and formaldehyde. These products made possible the near complete removal of all ions, including aqueous sodium chloride (xe2x80x9cNaCl(aq)xe2x80x9d), from water. However, the anion exchanger was unable to remove weak alkaline or silica based acids, and the important chemical species to accomplish the ion reduction were both expensive to produce and difficult to handle.
More recently, various desalinization systems have relied upon phase transformations, electrodialysis, or reverse osmosis techniques which are energetically expensive processes. A semi-permeable membrane, like the cell wall of a bladder, is selective about what it allows through it, generally allowing small molecules like water to pass easily but preventing the passage of many other compounds. With the presence of two solutions, each containing a different concentration of dissolved compounds on either side of the barrier, water will typically move from the side of the more dilute solution to the more concentrated solution. Eventually, osmotic pressure will counter the diffusion process exactly, and equilibrium will form. The process of reverse osmosis, first described by a French scientist in 1748, forces a net flow of water molecules from an aqueous solution with a greater concentration of compounds present within it through a semi-permeable membrane and into a solution with a lower concentration of dissolved compounds In a relatively expensive energy step, high water pressure on the source side is used to xe2x80x9creversexe2x80x9d the natural or forward osmotic process. For example, the desalinization of brackish water typically requires operating pressures within the range of 250 to 500 psi, while seawater desalinization requires operating pressures from 800 to 1000 psi, to obtain potable water as the final product.
Other workers in the field have attempted to provide workable desalinization systems to address the long standing need for an efficient and inexpensive method for the demineralization of water. See for example, Batchelder, U.S. Pat. No. 3,171,799; Glew, U.S. Pat. No. 3,216,930; Halff, U.S. Pat. No. 3,617,547; Frank, U.S. Pat. No. 3,670,897; and Yaeli, U.S. Pat. No. 5,098,575. However, none suggest the type of osmolar and thermal manipulations provided by the instant invention.
Through the use of the osmosis techniques and apparati of the invention described herein, virtually unlimited amounts of water can be made potable for a variety of uses including agricultural uses, commercial uses, and as a source of drinking water. More importantly, the creation of potable water through desalinization can essentially xe2x80x9cdrought-proofxe2x80x9d a given population, region, or industry.
The present invention teaches a method and apparatus for recovering water from aqueous solutions. In a preferred embodiment, the present invention teaches a forward osmotic process to create potable water from a saline solution through the manipulation of thermal and osmolar conditions of multiple solutions in a series of reactions which result in the passage of H2O from one solution to another through solvent transfer means, such as semi-permeable membranes.
The invention relates to an osmotic process or method for the extraction of a solvent from a first solution, having a first solute difficult to separate from the solvent, by passing said solvent through a series of intermediate solutions comprising the steps or drawing the first solution into a first heating means to heat the first solution; drawing the first solution into a first chamber that is divided by a first solvent transfer means from a saturated second solution having a second solute in a second chamber; subjecting the first solution to xe2x80x9cnaturalxe2x80x9d or forward osmosis by means of the first solvent transfer means, in opposition to the second solution such that a net osmotic flow of the solvent takes place across the first solvent transfer means and into the second solution thereby increasing the volume of the second solution; drawing the second solution from the second chamber into a cooling means to cool the second solution; drawing the second solution into a third chamber that is divided by a second solvent transfer means from a third solution having a third solute in a fourth chamber; subjecting the second solution to forward osmosis by means of the second solvent transfer means, in opposition to the third solution such that a net osmotic flow of solvent takes place across the second solvent transfer means and into the third solution thereby increasing the volume of the third solution; drawing the third solution from the fourth chamber into a second heating means to heat the third solution; drawing the third solution into a fifth chamber to provide for removal of any remaining third solute; and collecting the third solution, now diluted, for use. Optionally, the fifth chamber can be divided by a third solvent transfer means from a fourth solution having a fourth solute in a sixth chamber. Prior to collecting the third solution, it would be subjected to forward osmosis by means of the third solvent transfer means, in opposition to the fourth solution such that a net osmotic flow of the solvent takes place across the third solvent transfer means and into the fourth solution, increasing its volume and diluting it. The fourth solution would then be collected, now diluted, for use.
In this invention the flow of H2O from one opposing solution to another is such that there is a net osmotic flow of H2O from a saline solution, such as seawater, through a series of intermediate solutions to an ending solution. The intermediate solutions, which are subjected to thermal and osmolar manipulation, are used to generate a final solution with a very low concentration of an acceptable or desirable end-product solute. The process takes advantage of the solutes having highly temperature dependent solubilities, such as KNO3(s) and SO2(g), as well as the relatively temperature indifferent solubility of NaCl(aq), the primary solute present in seawater.
In general, the first solution is a saline solution and the second and third solutions are aqueous solutions of salts (i.e., water is the solvent and the salt is the solute) having temperature dependent solubility. The solubility of the second solute is directly related to temperature while the solubility of the third solute is inversely related to the temperature. A fourth, optional solution contains one or more desired additives that will be introduced into the final product. Typically the first solution is seawater (NaCl(aq), the second solution is a saturated KNO3 solution, the third solution is a saturated SO2 solution at 15 atm partial pressure to insure sufficient concentration of the SO2 solute, and the fourth solution is selected from the group of fertilizers such as NH4NO3, vitamins, minerals, NaF, chlorine or other water treatment additives.
To accomplish the goal of desalinization, a first solution (e.g. seawater) is drawn into a first heat exchanger, for example a steam condenser, where it is heated after which it is drawn into a first chamber. The first solution will typically have a temperature within the range of 20 to 80xc2x0 C., more typically about 50xc2x0 C., and will be heated to a temperature within the range of about 60 to 100xc2x0 C., more typically about 100xc2x0 C. It should be noted that the seawater drawn into the first heat exchanger can also be pre-heated to a desired temperature by first passing through a second heat exchanger where it removes heat from the KNO3 solution entering the second chamber. The pre-heated seawater would then enter the first heat exchanger. The first chamber is divided by a semi-permeable membrane from a saturated second solution of a soluble salt whose solubility increases with an increase in temperature and vice versa. Due to the higher relative concentration of solutes in the second solution a net osmotic flow of H2O takes place across the membrane from the first solution into the second solution. This second solution is then drawn into a heat exchanger where it is allowed to cool while transferring its heat to the incoming seawater, such that the salt precipitates out until the solution is maximally saturated at the lower temperature. The second solution is typically cooled to a temperature within the range of 20 to 40xc2x0 C., more typically about 30xc2x0 C. The heat xe2x80x9clostxe2x80x9d in this step may in fact be transferred to incoming seawater so as to be used more efficiently, as noted above. Thereafter the saturated second solution is drawn into a second chamber divided by another semi-permeable membrane from a third solution, under pressure, saturated with a solute whose solubility increases with a decrease in temperature. Due to the relatively greater saturation of the third solution as compared to the second solution, a net osmotic flow of H2O takes place across the second semi-permeable membrane from the second solution into the third solution. Thereafter the third solution is drawn into another heat exchanger where the solution is heated to a temperature within the range of 60 to 100xc2x0 C., more typically about 100xc2x0 C., accompanied by a corresponding decrease in the concentration of the solute. After being heated, the third solution is (with slight modification) the final product, or is drawn into another chamber divided by another semi-permeable membrane and opposed against a fourth solution of a desirable end product solute, which solute is added to generate the final product. A net osmotic flow of H2O takes place across the membrane towards the fourth solution, resulting in a treated fourth solution that is then ready for use as potable or agricultural water, as dictated by the nature of the end product solute.
In one embodiment of this invention, assistive pressure is introduced in the fourth or final stage such that the xe2x80x9cforwardxe2x80x9d osmotic nature of the reaction is retained while the required surface area of the semi-permeable membranes used can be reduced. Alternatively, this assistive pressure can be used to increase the system rate of flow. It may also be desirable to introduce assistive pressure to the process as a whole or at various other steps. As indicated, this use of positive pressure aids in the forward osmosis contemplated by the invention.
In another embodiment of the invention, the processes described above are modified to allow the generation of energy. This energy is used to offset any needed energy expenditures needed to heat a given solution or pressurize a gas. For example, as the volumes of solutions increase due to net osmotic flow, the solutions may be allowed to rise, by means of their high osmotic pressure differentials, to a height sufficient to allow the generation of electricity should the solutions be then allowed to flow through hydroelectric turbines designed for this purpose. The power generated in this way is then used to sustain the operation of a compressor and/or pump for the gaseous solute of the third solution, for example, for SO2(g).
This invention is designed primarily for use in a co-generation environment, using as its primary energy source the low pressure xe2x80x9cwastexe2x80x9d steam which has been used to turn steam-driven electric generators. Typically, this energy would be used to raise the temperature of a given solution to a desired level. Other energy sources are feasible for this use, including solar, wind, geothermal, and conventional means.
The end product of potable water is available, regardless of rainfall patterns and without reducing supplies of other users or supplies needed to protect aquatic ecosystems. Desalinization used in combination with traditional water projects allows more efficient use of low cost storage based systems, and can even be used to recharge aquifers. With a reliable reserve of potable water, water resources from traditional sources can be more fully utilized without fear of drought shortages, while minimizing losses to evaporation and spillage. This maximum use of low cost water results in financial benefits that can offset the typically high cost of desalination facility construction.
Crystalline precipitates of the saturated solutions disclosed by this invention are kept suspended in their respective solutions to allow ordered deposit of solid, and to maintain saturation. In the first stage of the process, crystalline NaCl would be removed periodically, as necessary. This by-product may be sold commercially or put to other desirable use. Other precipitates occurring as a result of osmolar or pressure manipulations are themselves removed or recycled into the system as necessary or desirable.
An object of the invention is to provide an improved and more economically, environmentally, and energetically efficient process for desalinization of seawater.